American Journal of Biochemistry and Biotechnology, 2012, 8 (2), 99-104 ISSN: 1553-3468 © 2012 Amri and Mamboya, This open access article is distributed under a Creative Commons Attribution (CC-BY) 3.0 license doi:10.3844/ajbbsp.2012.99.104 Published Online 8 (2) 2012 (http://www.thescipub.com/ajbb.toc) Papain, a Plant Enzyme of Biological Importance: A Review Ezekiel Amri and Florence Mamboya Department of Science and Laboratory Technology, Dar es Salaam Institute of Technology (DIT), P. O. Box 2958, Dar es Salaam, Tanzania Received 2012-05-13; Revised 2012-05-20; Accepted 2012-06-02 ABSTRACT Papain is a plant proteolytic enzyme for the cysteine proteinase family cysteine protease enzyme in which enormous progress has been made to understand its functions. Papain is found naturally in papaya ( Carica papaya L.) manufactured from the latex of raw papaya fruits. The enzyme is able to break down organic molecules made of amino acids, known as polypeptides and thus plays a crucial role in diverse biological processes in physiological and pathological states, drug designs, industrial uses such as meat tenderizers and pharmaceutical preparations. The unique structure of papain gives it the functionality that helps elucidate how proteolytic enzymes work and also makes it valuable for a variety of purposes. In the present review, its biological importance, properties and structural features that are important to an understanding of their biological function are presented. Its potential for production and market opportunities are also discussed. Keywords: Proteolytic enzyme, cysteine protease, papain, structure, hydrophobic 1. INTRODUCTION 1.1. Properties, Structure and Features of Papain Papain (EC 3.4.22.2) is an endolytic plant cysteine The globular protein, the papain PDB accession protease enzyme which is isolated from papaya ( Carica number 1CVZ is a single chain protein with molecular papaya L.) latex. Papain is obtained by cutting the skin weight of 23,406 DA and consists of 212 amino acid of the unripe papaya and then collecting and drying the latex which flows from the cut. The greener the fruit, with four disulfide bridges and catalytically important more active is the papain. Papain enzyme belongs to the residues in the following positions Gln19, Cys25, His158 papain superfamily, as a proteolytic enzyme, papain is of and His159 (Mitchel et al ., 1970; Robert et al ., 1974; crucial importance in many vital biological processes in Tsuge et al ., 1999). The graphical representation of the all living organisms (Tsuge et al ., 1999). Papain shows amino acid composition of papain is shown in Fig. 1 . extensive proteolytic activity towards proteins, short- Papain is a cysteine hydrolase that is stable and active chain peptides, amino acid esters and amide links and is under a wide range of conditions. It is very stable even at applied extensively in the fields of food and medicine elevated temperatures (Cohen et al ., 1986). Papain is (Uhlig, 1998). It preferentially cleaves peptide bonds unusually defiant to high concentrations of denaturing involving basic amino acids, particularly arginine, lysine agents, such as, 8M urea or organic solvent like 70% and residues following phenylalanine (Menard et al ., EtOH. Optimum pH for activity of papain is in the 1990). The unique structure of papain gives its range of 3.0-9.0 which varies with different substrate functionality that helps to understand how this (Edwin and Jagannadham, 2000; Ghosh, 2005). proteolytic enzyme works and it’s useful for a variety Papain enzyme as cysteine proteases in papain of purposes. This review addresses structural features superfamily is usually consisting of two well-defined of enzyme, the biological importance and processes in domains which provide an excellent system for studies which papain participates and its potential for in understanding the folding-unfolding behavior of production market opportunities. proteins (Edwin et al ., 2002). Science Publications 99 AJBB Ezekiel Amri and Florence Mamboya / American Journal of Biochemistry and Biotechnology 8 (2) (2012) 99-104 Fig. 1. Graphical representation of the amino acid composition of papain The protein is stabilized by three disulfide bridges in which the molecule is folded along these bridges creating a strong interaction among the side chains which contributes to the stability of the enzyme (Edwin and Jagannadham, 2000; Tsuge et al ., 1999). Its three-dimensional structure consists of two distinct structural domains with a cleft between them. This cleft contains the active site, which contains a catalytic diad that has been likened to the catalytic triad of chymotrypsin. The catalytic diad is made up of the amino acids-cysteine-25 (from which it gets its classification) and histidine-159. Aspartate-158 was thought to play a role analogous to the role of aspartate in the serine protease catalytic triad, but that has since then been disproved (Menard et al ., 1990). Fig. 2. Papain structure (MMDB protein structure summary, Papain molecule has an all-α domain and an 1CVZ) antparallel β-sheet domain (Kamphuis et al ., 1984; Madej et al ., 2012). The conformational behavior of papain in aqueous solution has been investigated in the presence of SDS and reported to show high α-helical content and unfolded structure of papain in the presence of SDS is due to strong electrostatic repulsion (Huet et al ., 2006). In the molten globule state (pH 2.0), papain show evidence of substantial secondary structure as ß-sheet and is relatively less denatured compared to 6 M Guanidium Hydroc8hloride (GnHCl), the enzyme also exhibits a great tendency to aggregate at lower concentrations of GnHCl or a high concentration of salt (Edwin and Jagannadham, 2000). Papain apart from being most studied plant cysteine proteases, further researches in understanding the specificity, the structural the effect brought by inhibitors, low pH, metal ions and fluorinated alcohols has been identified as of critical Fig. 3. Hydrophobic amino acid of papain (1CVZ). Colored gray importance (Huet et al ., 2006; Naeem et al ., 2006). in a space fill model are the backbone oxygen and nitrogen Figure 2 shows the structure of papain from Molecular of the residues with the hydrophobic side chain (Source: Modeling Database (MMDB), the structure is shown http://www.oocities.org/bramsugar/intro4.html). with all-α domain and an antparallel ß-sheet domain. Science Publications 100 AJBB Ezekiel Amri and Florence Mamboya / American Journal of Biochemistry and Biotechnology 8 (2) (2012) 99-104 1.2. Hydrophobicity of Papain 1.3. Mechanism, Biological Importance and Functions It is often useful to examine the relative hydrophobicity or hydrophilicity values of the amino 1.4. Mechanism of Functions acids in a protein sequence. Since hydrophobic residues The mechanism in which the function of papain is tend to be more buried in the interior of the molecule and made possible is through the cysteine-25 portion of the hydrophilic residues are more exposed to solvent, a triad in the active site that attacks the carbonyl carbon in profile of these values can indicate the overall folding the backbone of the peptide chain freeing the amino pattern. The hydrophobic interactions have the major terminal portion. As this occurs throughout the peptide influence in protein conformation and the most chains of the protein, the protein breaks apart. The hydrophobic of the amino acid side chains are those of mechanism by which it breaks peptide bonds involves alanine, Valine, leucine, methionine and Isoleucine deprotonation of Cys-25 by His-159. Asparagine-175 which vary in degrees of hydrophobic. The hydrophobic helps to orient the imidazole ring of His-159 to allow -hydrophilic interaction of papain amino acids in the side this deprotonation to take place. Although far apart chain seems to be the major thermodynamic forces within the chain, these three amino acids are in close which drive protein folding. Investigation of the proximity due to the folding structure. It is though these formation of the intermediate state of papain through three amino acids working together in the active site that inducing n-alkyl sulfates including sodium octyl sulfate, provides this enzyme with its unique functions. Cys-25 SOS; sodium decyl sulfate, SDeS; and sodium dodecyl then performs a nucleophilic attack on the carbonyl sulfate, SDS at different concentrations has exhibited carbon of a peptide backbone (Menard et al ., 1990; that hydrophobic interactions play an important role in Tsuge et al ., 1999). In the active site of papain, Cys -25 inducing the two different intermediates along the two and His -159 are thought to be catalytically active as a various thermodynamic pathways (Chamani et al ., thiolate-imidazolium ion pair. Papain can be efficiently 2009). Catalytic activity of papain involves hydrolysis of inhibited by peptidyl or non-peptidyl N-nitrosoanilines proteins with broad specificity for peptide bonds, but (Guo et al ., 1996; 1998). The inactivation is due to the preference for an amino acid bearing a large hydrophobic formation of a stable S-NO bond in the active site ( S- side chain at the P2 position while does not accept Val in nitroso-Cys 25 ) of papain (Xian et al ., 2000). P1 (Kamphuis et al ., 1985). The enzyme has been reported to be generally more stable in hydrophobic 1.5. Papain in Medical Uses solvents and at lower water contents and can catalyze Papain acts as a debris-removing agent, with no reactions under a variety of conditions in organic harmful effect on sound tissues because of the enzyme’s solvents with its substrate specificity little changed from specificity, acting only on the tissues, which lack the α1- that in aqueous media (Stevenson and Storer, 1991). In antitripsine plasmatic antiprotease that inhibits proteolysis general, native proteins have a hydrophobic core and a in healthy tissues (Flindt, 1979). The mechanism of charged and/or polar group on the surface. The biochemical removal of caries involves cleavage of hydrophobic core helps to stabilize the tertiary structure of polypeptide chains and/or hydrolysis of collagen cross- the protein by hydrophobic interaction while the outer linkages.